Clams, Class: Bivalvia, Representative mollusks. Bivalves have a shell with two halves. Filter feeders, they take in food and water through a tubular siphon. In the clam, a muscular foot is used for burrowing and creeping. A sea shell is the most universally identifiable part of a creature known as a mollusk. Mollusks (Sea Shells) are invertebrate animals (think of a snail) with an un - segmented, basically symmetrical body, generally consisting of head, foot, visceral hump and mantle. Mollusks (Sea Shells) are descendants from primitive wormlike creatures inched around in the ooze of primeval seas millions of years ago. As dissolving land masses began to feed salts and chemicals into oceans, the first Mollusks (Sea Shells) digested them and eventually used them to build durable shelters.